Implantable prostheses are commonly used to replace or augment body tissue. In the case of breast cancer, it is sometimes necessary to remove some or all of the mammary gland and surrounding tissue, creating a void that can be filled with an implantable prosthesis. The implant serves to support surrounding tissue and to maintain the appearance of the body. The restoration of the normal appearance of the body has an extremely beneficial psychological effect on post-operative patients, eliminating much of the shock and depression that often follows extensive surgical procedures. Implantable prostheses are also used more generally for restoring the normal appearance of soft tissue in various other areas of the body, such as the buttocks, chin, calf, etc.
Soft implantable prostheses typically include a relatively thin and quite flexible hollow envelope or shell made of vulcanized (cured) silicone elastomer. The shell is filled either with a silicone gel or with a normal saline solution. The filling of the shell takes place before or after the shell is inserted through an incision in the patient.
Traditional molding of implantable breast implant shells involves covering a mold (more typically called a mandrel) in uncured silicone dispersion through immersion into baths or by passing through a curtain of silicone dispersion and allowing the dispersion to flow over the mandrel just by utilizing gravimetric forces. Because the common method was to immerse or dip the mandrel into a bath of silicone dispersion, the process is commonly referred to as “dip-molding.” Other more recent developments are the use of spraying the dispersion onto mandrels, or by using rotational molding techniques. Whereas silicone (i.e., polysiloxane, a polymer in which the main chain consists of alternating silicon and oxygen atoms with organic side groups) is the most common material of construction, other materials such as polyurethane have been used.
FIGS. 1A-1C illustrate one early process for dip-molding flexible implant shells for implantable prostheses and tissue expanders, involving dipping a suitably shaped mandrel 20 into a silicone elastomer dispersion 22 of a silicone solution and a solvent. The mandrel 20 is withdrawn from the dispersion and the excess dispersion is allowed to drain from the mandrel. After the excess dispersion has drained from the mandrel at least a portion of the solvent (typically xylene) is allowed to evaporate to stabilize the silicone elastomer coating, forming a gum state. The process is then repeated several times until a shell of the desired thickness is formed. The layered structure characteristic of some silicone elastomer shells can be made by sequentially dipping the mandrel in different dispersions.
FIG. 2 illustrates an exemplary cross-section of a basic gel-filled breast implant 28, in which a shell 30 is initially formed by a process such as the above-described mandrel dipping process. The outer silicone elastomer shell 30 has an anatomical configuration, in this case matching the breast, and comes off a mold with a shell hole 32. In the illustrated embodiment, a patch over the shell hole 32 includes an uncured portion 34 directly over the hole and a cured portion 36 covering that and adhered to the inner surface of the shell 30. The patch is cured and then the hollow interior of the shell 30 is filled with an appropriate gel 38, such as via a needle hole in the patch. The silicone gel is supplied as a two-part liquid system with a primary gel component and a cross-linking component. The needle hole in the patch is then sealed with a silicone adhesive or plug, and the implant oven cured to achieve cross-linking of the gel.
Variations on the basic dip-molding technique are available. For example, U.S. Patent Publication No. 2004/0245671 to Smit discloses an automated system for dip molding silicone covers (shells) for breast implants in which an arm positions a mold to immerse an upper portion prior to total immersion thereof, ostensibly to eliminate air inclusions in the molded product. The system includes, in sequence, a cleaning station, a dipping station, an evaporating oven, and a curing oven. U.S. Patent Publication No. 2008/0208336 to Job includes the technique of spraying the silicone dispersion onto mandrels, one embodiment of which results in a nonuniform shell thickness.
Another process for forming implant shells is rotational molding, such as the system and methods described in U.S. Pat. No. 6,602,452 to Schuessler. The process also results in a flexible implant shell having a hole that requires a patch. Rotational molding provides a number of advantages to dip- or spray-molding, though the latter are currently more common in the industry.
Despite many advances in the construction of soft prosthetic implant shells, there remains a need for a simpler process that results in a more consistently uniform implant shell thickness.